1. Technical Field
This invention relates to the fabrication of semiconductor detector devices. More particularly, it relates to a method of utilizing selective thinning techniques and two-sided wafer processing to define a detector device.
2. Background
Thinned semiconductor detector devices are currently gaining wide acceptance in a variety of applications, particularly in applications requiring reduced detector volume to optimize radiation hardness characteristics. Performance characteristics of these devices generally depend on a number of factors including: uniform ohmic contacts over detector surfaces, well defined photosite aperture, and reliable ohmic contact of the detector bias wire bound to the light shield side of the detector. To achieve such high performance characteristics in detector devices, production methods require the ability to easily handle a thinned semiconductor substrate so that high temperature processes such as furnace annealing, photolithographic processing, and metal vapor deposition can easily be applied to both sides of the thinned device. Unfortunately, it has been extremely difficult to perform two-sided processing on thinned semiconductor substrates to fabricate these detector devices.
In recent years, the conventional approach to fabricating thinned detector devices has been to process one side of the device, attach the substrate to a support, mechanically thin the opposite side of the device, and complete processing of the device on the thinned side. That method has typically been approached by first implanting ions, annealing the implanted layer and depositing the patterning aluminum and indium bump layers to form part of the detector on a silicon substrate material. The processed surface of the substrate is then encapsulated to protect it during subsequent processing. The substrate is then attached to a silicon backing wafer to cover the processed side of the substrate. The backing wafer is then used as a means for handling the substrate during a subsequent thinning process. Thinning is achieved by lapping and then chemically polishing the substrate to a desired uniform thickness. With the backing wafer still attached, the thinned surface of the substrate is further processed to complete the detector device.
Unfortunately, this technique has several drawbacks. In particular, the presence of a backing wafer throughout most of the processing stages severely limits processing after the substrate has been thinned. In particular, the presence of the backing wafer does not generally allow for high temperature processes to be applied to the device. As known, many routine silicon processes, such as implant annealing and metal sintering are conducted at high temperatures. Consequently, under this technique, implants have required laser surface annealing, which tends to produce poor implant annealing characteristics. Such characteristics include the formation of nonuniform contact resistances, which can harm the performance of the detector. Additionally, the presence of the backing wafer does not readily permit fabrication of a light shield, which is normally used in these infrared detector devices. Finally, upon detachment of the backing wafer, the resultant fragile thinned device has posed handling risks during hybridization processes. The result of the foregoing has been low yield statistics of performing devices.
The use of chemical etching techniques has been recorded in the literature as an alternative means for thinning semiconductor substrates. See, for example, Varker et al., "Preparation of Large-Area, Electron-Transparent Silicon Specimens by Anisotropic Etching", Solid State Technology, April, 1983, pg. 143. However, the literature does not provide a teaching as to how to overcome one or more of the problems discussed above.